On 3/7/2013 5:14 PM, Tim Williams wrote:
wrote in message
...
High Q isn't the goal, high radiation resistance is -- the bigger the
loop, the better it couples with free space, until it's a wave length
around.
I'm not clear on why you keep referring to radiation resistance for a
receiving antenna. Does this result in a larger received signal? I am
concerned with maximizing the voltage at the input to the receiver.
You're also not concerned about that -- you're concerned about maximizing
SNR at the receiver.
SNR would be good, but I am concerned with maximizing the signal actually.
A Q of a million will get you gobs of "gain", but if it doesn't couple
into free space, it's only the thermal noise of the loss generating that
signal.
I think you aren't reading what I am writing. I said I wanted a Q over
100, not 1 million. I don't get why you keep talking in hyperbole.
What you are describing is not even a tradeoff between signal strength
and SNR. If there is no coupling, there is no signal.
An antenna with high (expressed as ESR) radiation resistance might have a
modest Q, but gives far better SNR because it couples to free space.
I have not found anything to indicate this produces a better receive
antenna. I have a formula for the effective height of a loop antenna
which is what determines the received signal strength at the antenna. It
does not calculate the radiation resistance, it uses the coil parameters
and the wire resistance. Is that a wrong formula?
Raw volts don't matter, you can always throw more amplifiers at it (as
long as they don't corrupt the SNR also!).
Maybe you didn't read my other posts. I am not using an amplifier. I
am running the antenna and coupler output directly into a digital input.
Yes, a higher stepup ratio gets larger signal up to a point. That point
is determined by the parasitic capacitance of the receiver input. That
capacitance is reflected back through the transformer and affects the
antenna tuning. In my simulations it creates a filter with two
resonances.
Oooh, capacitance! I like capacitance. Capacitance is easy to
cancel...inductors are good at that. 
What's a nearby inductor working against that capacitance? The current
transformer in your simulation, if its inductance can be controlled, would
be an excellent candidate. The circuit effectively becomes a double tuned
interstage transformer, like,
http://www.jrmagnetics.com/rf/doubtune/doubccl_c.php
This is two resonators coupled with a cap, but any coupling method will
do. Capacitive, magnetic (putting the coils end-to-end) or
electromagnetic (coils side-by-side) coupling does equally well; normal
arrangements have them all in phase, so in practice, unshielded coils will
need smaller coupling capacitance than designed, etc.
If you line up that 10p resonance with the operating frequency, you should
get gobs more gain. In fact, because the reactances cancel, the driven
impedance will be much higher than you were expecting, and so will the
gain. The CT might go from, say, 1:8 up to, who knows, 1:20? 1:100?
The bandwidth of that coupling (not necessarily of the antenna itself, so
they should be similar bandwidths) is determined by the coupling
coefficient (in the coupled-inductors case, simply k) and Q of the
components.
If your receiver datasheet specifies an equivalent input circuit, you
might be able to estimate the equivalent loss and optimize gain.
The receiver input is high impedance, approximately 10 MOhms with a low
capacitance between the differential inputs of not more than 10 pF.
Your description of what is happening is very terse and full of
shortened terms that I don't understand. What do you mean "line up that
10p resonance with the operating frequency"? I assume you are referring
to the 10 pF input capacitance. How does this get "lined up" with
anything?
When you talk about reactances canceling, that sounds a lot like a tuned
circuit at resonance. That is what I *am* doing and where this thread
started. One problem with that is the lack of precision or stability of
the parasitic capacitance. Any idea how to deal with that?
Have you looked at the simulation data I had posted? I think you are
describing exactly the circuit we are simulating which I believe is an
accurate representation of the circuit I plan to build. Is that not
correct?
--
Rick